sort time units

this patch sort the generated OnCalendar and makes it more readable.

Example:
>Description=[Cron] "*/5 * * * * munin if [ -x /usr/bin/munin-cron ]; then /usr/bin/munin-cron; fi"                            
                                                                                                                              
Before
>OnCalendar=*-*-* *:0,35,5,40,10,45,15,50,20,55,25,30:00 

After
>OnCalendar=*-*-* *:0,5,10,15,20,25,30,35,40,45,50,55:00